The pneumococcal capsule has always been described as the major virulence factor of Streptococcus pneumoniae. The capsule is thought to function by masking cell wall components that are easily recognized by antibody and complement in normal serum. In this grant we propose a continuation of ongoing studies concerning the importance of the pneumococcal surface protein A(PspA) (67-99 kd) in pathogenesis, and its potential as an immunity producing antigen. PspA appears to be quite variable antigenically, somewhat reminiscent of the variation observed in the antigenic structure of the pneumoccal capsule. We have shown previously that two monoclonal antibodies to PspA can protect mice from fatal infection with some of the strains with which the antibodies react. More recently we have used insertional inactivation to produce mutants of pneumococci lacking PspA. By using these mutations we have made two important observations. In one we found that immunization of xid mice (which are genetically unable to respond to polysaccharides) with heat-killed PspA+ pneumococci results in greater protection against challenge with virulent pneumococci than immunization with heat-killed PspA- pneumococci. Thus it appears likely that PspA may be able to be used as a protection eliciting immunogen. Our other observation has been that the absence of PspA can have an effect on virulence of pneumococci, which in some strains (including the most mouse virulent strain we have) has been as large as that of the capsule itself. Our blood clearance studies indicate that this decrease in virulence is associated with a surprising increase in the susceptibility of the strains to complement dependent killing. This latter finding was quite unexpected, but may indicate that the resistance of pneumococci and other Gram positive organisms to complement dependent killing may be the result of surface proteins such as PsPA. In our proposed studies we hope to: 1) determine the importance of PspA as a virulence factor in a large number of pneumococcal isolates, 2) to determine the mechanism by which it interferes with complement mediated killing of pneumococci, 3) determine whether PspA's isolated from a number of different pneumococcal strains can elicit protective immune responses, 4) clone the entire PspA gene, and 5) identify the most immunogenic epitopes of PspA.